1. Field of the Invention
The present invention relates to a distributed feedback (DFB) semiconductor laser and a method of manufacturing the same.
2. Related Background Art
A distributed feedback (DFB) semiconductor laser is used as a light source for an optical pickup device in an optical recording/reproducing apparatus in which an information signal is recorded/reproduced on/from an optical disc. The DFB semiconductor lasers are known as devices which can be applied in the fields of optical communications systems such as optical CATVs, pumping light sources for small solid-state lasers or second harmonic generation (SHG) short-wavelength lasers for high-density information recording, and optical measurement systems.
Electrons and holes as well as emitted lightwaves in the semiconductor laser should be confined within a small region of the semiconductor in order to obtain a transverse single-mode oscillation and low power operation. For this purpose, generally, a so-called double-heterostructure (DH structure) is employed in a direction perpendicular to the pn-junction of the semiconductor laser. With regard to a horizontal direction, a gain-guided or an index-guided structure is commonly used as a waveguide. The index-guided structure has a configuration in which an emission region of the laser is surrounded by a cladding material with a low refractive index in the horizontal direction as well as in the direction perpendicular to the pn-junction of the semiconductor laser. The index-guided structure can be realized in a laser with the emission region, for example, in a ridge in the form a stripe.
The ridge in the form of a stripe (hereinafter, simply referred to as a ridge) is formed by etching off the portion of a cladding layer other than a current injection region. Confinement of a lightwave can be attained in the ridge structure laser, since the effective refractive index for the lightwave propagating in a guide layer beneath the ridge is greater than the other portion. The ridge structure laser can be oscillated by injecting a current from an electrode provided on the top of the ridge to a substrate.
A surface grating DFB laser having an asymmetric cladding waveguide structure is proposed, as described in IEEE Photonics Technology Letters, Vol.9, No. 11, November 1997, pp.1460-1462.
FIG. 1 is a schematic cross-sectional view of an asymmetric cladding DFB laser. The asymmetric cladding DFB laser has the following features.
First, the asymmetric cladding DFB laser is provided with cladding layers 1, 3 and an active layer 2 which is sandwiched between the cladding layers 1 and 3. The thickness of the upper cladding layer 3 is made smaller than that of the lower cladding layer 1 (i.e., an asymmetric cladding layer structure), so that the intensity of the optical field becomes asymmetric with regard to the direction perpendicular to the waveguide as indicated by a dashed line.
Second, the optical field of the laser emission penetrates into the grating region, since the upper cladding layer 3 is thinner than the lower cladding layer 1. The grating region includes insulating segments 6, which are periodically placed on the upper cladding layer 3, and a metal electrode 7. The insulating segments 6 and the metal electrode 7 may be made of SiO2 and Tixe2x80x94Ptxe2x80x94Au, respectively. Therefore, the emitted light is modulated by the grating to bring about a single-longitudinal-mode oscillation. The asymmetric cladding DFB laser is one of gain-coupled DFB lasers, since the optical absorption coefficients of the insulating segments 6 (SiO2) and the metal electrode 7 (Tixe2x80x94Ptxe2x80x94Au) are different.
As mentioned above, the optical absorption due to the metal electrode 7 is utilized in the asymmetric cladding DFB laser. On the other hand, the utilization of the optical absorption has the adverse effect of decreasing the slope efficiency (i.e., a gradient of output power versus injection current) of the laser. A region indicated by xe2x80x9cAxe2x80x9d in FIG. 1 shows an absorption region due to the metal electrode 7.
More particularly, the optical absorption by the metal below the top surface 6a of the insulating segments 6 is necessary for single-longitudinal-mode oscillation, while the optical absorption by the metal above the top surface 6a is unnecessary. The greater absorption within the grating region degrades the laser performance.
The present invention is made in consideration of the above circumstances and it is an object of the present invention to provide a DFB semiconductor laser and a method of manufacturing the laser wherein an optical absorption loss by a metal electrode formed on a grating can be avoided.
According to the present invention, there is provided a method of manufacturing a ridge-structure DFB semiconductor laser, which includes the steps of: sequentially forming an active layer, a cladding layer and a contact layer on a substrate; partially removing the cladding layer and the contact layer to a predetermined depth to form flat portions and a ridge protruding from the flat portions; forming a plurality of metal strips having a predetermined periodicity along a longitudinal direction of the ridge and extending from a surface of at least one of the flat portions to a top of the ridge; forming an insulating layer on the plurality of metal strips, the at least one of the flat portions and the top of the ridge; removing a portion of the insulating layer on the at least one of the flat portions; and forming an electrode electrically connected to the plurality of metal strips.
According to another aspect of the present invention, the electrode is only formed on the at least one of the flat portions.
According to still another aspect of the present invention, the electrode is formed on the insulating layer so as to extend from the top of the ridge to the plurality of metal strips on the at least one of the flat portions.
According to further aspect of the present invention, the active layer is an In1xe2x88x92xGaxAs1xe2x88x92yPy (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) bulk layer or In1xe2x88x92xGaxAs1xe2x88x92yPy (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) single- or multiple-quantum-well layer, the cladding layer is an InP layer, and the contact layer is an In1xe2x88x92xGaxAs1xe2x88x92yPy (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) layer or an In1xe2x88x92xGaxAs (0xe2x89xa6xxe2x89xa61) layer.
According to still further aspect of the present invention, the step of forming a plurality of metal strips includes the steps of: forming a metal film and subsequently a photoresist over the top of the ridge and the at least one of the flat portions, making a grating pattern of the photoresist having a predetermined periodicity along the longitudinal direction of the ridge, forming a plurality of metal strips having the predetermined periodicity and extending from a surface of at least one of the flat portions to a top of the ridge by using the grating pattern of the photoresist as a patterning mask.
According to another aspect of the present invention, the step of making a grating pattern includes the step of performing an electron-beam lithography.
According to the present invention, there is provided a ridge-structure DFB semiconductor laser, which includes: a ridge protruding from flat portions of cladding layer which is formed on an active layer, the ridge includes a cladding layer and a contact layer sequentially formed on the active layer; a plurality of metal strips having a predetermined periodicity along a longitudinal direction of the ridge and extending from a surface of at least one of the flat portions to a top of the ridge; and an insulating layer formed on the plurality of metal strips at the top of the ridge.
According to another aspect of the present invention, the laser further includes an electrode electrically connected to the plurality of metal strips.
According to still another aspect of the present invention, the electrode is only formed on the at least one of the flat portions.
According to further aspect of the present invention, the electrode is formed on the insulating layer so as to extend from the top of the ridge to the plurality of metal strips on the at least one of the flat portions.
According to still further aspect of the present invention, the active layer is an In1xe2x88x92xGaxAs1xe2x88x92yPy (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) bulk layer or In1xe2x88x92xGaxAs1xe2x88x92yPy (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) single- or multiple-quantum-well layer, the cladding layer is an InP layer, and the contact layer is an In1xe2x88x92xGaxAs1xe2x88x92yPy (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) layer or an In1xe2x88x92xGaxAs (0xe2x89xa6xxe2x89xa61) layer.